This invention relates to a graded composition primer layer for improving the bonding characteristics of a conductor to a substrate and to methods therefor.
Flexible "printed" circuitry is useful in the fabrication of lightweight electronic assemblies. Typically, a thin foil of a conductor such as copper is carried on a substrate made of a dielectric material, such as polyimide. Conductor-on-substrate assemblies are also used in rigid circuit boards and integrated semiconductor chip carriers.
It is known to fabricate conductor-on-substrate assemblies by adhesively laminating a thin foil of the conductor onto the substrate. However, adhesively bonded assemblies are performance-limited in a number of respects. The adhesive's outgassing and physical limitations (e.g., brittleness at low temperatures, loss of mechanical properties or instability at high temperatures) may preclude use of these assemblies in demanding environments, such as those involving exposure to high vacuum, high humidity, and/or temperature extremes. Dimensional variations in the adhesive caused by temperature fluctuations may be unacceptable where dimensional tolerances are critical.
It has been proposed to prepare adhesiveless assemblies by direct deposition of the conductor onto the substrate by electroless deposition or vacuum deposition, leading to a direct conductor-to-substrate bond. With either deposition method, the strength and durability of the bond between the conductor and the substrate is critical to the integrity of the product. In electroless deposition, aggressive chemical treatment is used to promote adhesion and prepare the surface for plating.
Vacuum deposition techniques represent an alternative to electroless deposition. Magnetron sputtering is a preferred vacuum deposition technique because high deposition energies and good bond strengths can be attained. The conductor may be sputtered directly onto the substrate. In some applications, it may be desirable to use a primer or adhesion promoting layer between the conductor and the substrate. For example, it has been proposed to use a primer layer of chromium between the copper conductor and polyimide substrate. It is believed that the chromium enables the formation of a stronger adhesive bond to the polyimide surface, preventing attack on the interface by plating and etching chemicals used in subsequent processing.
However, the primer layer itself can induce failure of a different nature, again resulting in limitations in performance capabilities. For example, a copper/chromium/ polyimide construction prepared by magnetron sputtering was found to have an excellent chromium-to-polyimide bond. However, upon testing in a 60.degree. C./90% relative humidity environment, failure at the copper/chromium interface was observed, although the chromium/polyimide bond appeared to remain intact. It is believed that this failure is attributable to the mutual insolubility of copper and chromium under the test conditions, so that there was oxidation of the chromium and consequent failure of the bond to the copper.
It has also been proposed to solve the problem of interfacial debonding resulting from oxidation of the adhesion layer by using Nichrome (an 80:20 Ni:Cr alloy) as an adhesion layer for copper. While the Nichrome layer does improve the adhesion of the copper, Nichrome is incompatible with certain plating chemistries and requires non-standard etchants for etching it, and is therefore not generally favored.